Hypertrophic localized mononeuropathy is a condition that comes to clinical attention as a painless focal swelling of a peripheral nerve in an arm or leg and is associated with a slow but progressive loss of motor and sensory function. Whether the proliferation of perineurial cells is neoplastic or degenerative--an ongoing controversy among nerve pathologists--for some patients resection of the involved portion of a nerve with autologous interposition grafting results in better functional outcome than allowing disease to follow its natural course. Patients with a painless focal enlargement of a nerve associated with progressive weakness and/or sensory loss may benefit from surgery for resection and grafting.